(1) Field of the Invention
This invention relates to a process for quenching a reaction product gas, which has been obtained by subjecting isobutylene or the like to vapor-phase catalytic oxidation, so as to recover methacrolein and/or methacrylic acid, and in particular to a quenching process and also to a reasonable recovery process from the liquid thus quenched.
(2) Description of the Prior Art
A quench column is generally used for collecting methacrolein and/or methacrylic acid from a reaction product gas which has been obtained by subjecting at least one compound selected from isobutylene, tertiary butanol, isobutyl aldehyde and methacrolein to catalytic vapor-phase oxidation with molecular oxygen in the presence of steam in accordance with a one-step or two-step reaction. As the manner of gas-liquid contact in such a quench column, there are two methods, one being counter current contact and the other parallel flow contact. As liquid compounds usable for these contacts, may be mentioned a reaction-gas condensate, benzene, benzene derivatives substituted by one or more alkyl groups having 1-4 carbon atoms, alkoxyl groups and/or alkoxycarbonyl groups aliphatic hydrocarbons having 5-7 carbon atoms, alicyclic hydrocarbons, etc. The reaction product gas however contains, in addition to methacrolein and/or methacrylic acid as a target product, high boiling byproducts such as benzoic acid, toluic acid, maleic acid, citraconic acid, terephthalic acid and tar-like substances at rather high concentrations. A trouble may hence arise that these high boiling byproducts may precipitate in the course of cooling of the reaction product gas and could hence block pipe lines.
A variety of methods have therefore been proposed for the prevention of blocking of pipe lines, including by way of example (1) to maintain the reaction product gas at a temperature at least equal to the boiling point of maleic anhydride under the pressure of the reaction product gas and further to control the average linear velocity of the reaction product gas at 5 m/sec or higher (Japanese Patent Laid-Open No. 126605/1975), (2) to control the flow velocity of the reaction product gas at 10 m/sec or higher at its feeding port of a quench column and to bring the reaction product gas into parallel flow contact with a condensate (Japanese Patent Laid-Open No. 91944/1982), (3) to maintain the temperature of the reaction product gas at 130.degree. C. or higher at the inlet of a scrubber (Japanese Patent Laid-Open No. 122327/1981), (4) to bring a gaseous reaction mixture into direct counter current contact at a temperature not higher than 100.degree. C. with a portion of a condensate condensed and accumulated in advance (Japanese Patent Laid-Open No. 52027/1979), etc.
Although these methods are effective for the prevention of blocking of pipe lines as stated in their specifications, they cannot still be considered as fully effective methods. Namely, the use of these methods caused such a problem that a localized temperature drop occurred at an inlet of a quench column due to conduction of heat to surrounding elements of structure upon feeding the reaction product gas into the quench column or splashes of a condensate were concentrated at the tip of a gas flow-out portion to result in the deposition of high boiling byproducts when the condensate was fed as a parallel flow. Once the deposition of high boiling byproducts takes place as described above, the byproducts are impregnated with methacrolein and methacrylic acid contained in the reaction product gas. These compounds have poor thermal stability. They therefore undergo polymerization there and become thicker gradually, so that the pipe line is blocked.
It has been known to guide the reaction product gas to a collector, the temperature of which is maintained above the dew point of the gas but below 250.degree. C., before condensing the gas (Japanese Patent Laid-Open No. 52239/1983), whereby high boiling byproducts are removed so as to improve such a drawback. In this method, high boiling and melting impurities contained in a gaseous form in the reaction product gas are forced to deposit for their removal However, the impurities thus deposited are composed of various substances. It is hence extremely difficult to remove the impurities completely within a similar temperature range. Many practical problems are also involved regarding the removal of the impurities thus precipitated and deposited.
The present inventors have already proposed to bring ammonia gas or ammonium hydroxide together with the condensate of the reaction product gas into contact with the reaction product gas so as to quench the reaction product gas (Japanese Patent Laid-Open No. 438/1987). This method is very effective in preventing the deposition of polybasic organic acids, e.g., terephthalic acid, as high boiling byproducts and also in preventing the polymerization of methacrolein and methacrylic acid. However, the ammonium salts of the above organic acids are partitioned to the side of an extraction residue in the subsequent extraction step and hence constitute a portion of an effluent. It is thus necessary to take a special measure for their treatment. Taking all the above features into consideration, the above method cannot be considered to be preferred fully.
It has also been attempted to solve the blocking of a pipe line by making improvements in equipment. In Japanese Patent Laid-Open No. 91944/1982 referred to above, the reaction product gas is introduced into a bottom portion of a quench column through a feed pipe provided at a right angle relative to a wall of the column, is blown against spreading buffle plates arranged in the form of a turned square U, and after being spread in horizontal directions, is allowed to flow upwards toward the top of the column. On the other hand, a portion of a condensate which has been cooled by a heat-exchanger is caused to fall as a shower along with a polymerization inhibitor from the top of the column, whereby the portion of the condensate is brought into counter current contact with the reaction product gas and the reaction product gas is hence quenched and condensed. In the above process, the cooled condensate which flows downwardly from the point above the spreading plates hits the spreading plates so that the spreading plates are cooled. The reaction product gas hence hits the spreading plates and is thus cooled, thereby resulting in condensation and coagulation of high boiling byproducts. Since the insides of the spreading plates are not exposed directly to the condensate containing the polymerization inhibitor, the condensation, coagulation and polymerization of the high boiling byproduct gas proceed at a tip portion of the feed pipe and the operation becomes no longer feasible eventually.
The above patent publication also discloses to introduce the reaction product gas and condensate from a top portion of a quench column, so that they are contacted to each other as parallel flows. In this method, the wall of the top portion is heated and due to conduction of heat, the lower wall is also exposed to high temperatures. The condensate supplied as the parallel flow splashes against the wall of the column The condensate is thus concentrated, leading to deposition of high boiling byproducts and polymerization of methacrolein and methacrylic acid.
In most of the above-described methods or processes for the quenching of the reaction product gas, it is proposed to condense the reaction product gas in a single step or multiple steps, generally at a temperature of 100.degree. C. or lower. Very broad temperature ranges are only referred to. The polymerization of methacrylic acid and the like as well as the formation of a solid matter from terephthalic acid and the like are especially serious problems as described above. The former problem may generally be solved when the temperature is controlled as low as possible. The latter problem, especially, a solid matter spread and suspended in a vapor phase cannot however be removed by scrubbing in a conventional tray column or packed column. No sufficient technique has been known for the prevention of occurrence of such polymerization or solid formation. It is hence dominantly practiced to collect and remove solid matters, which are contained in a gas from a quenching step of the reaction product gas or a methacrolein absorption step, by means of a cyclon or a scrubber of the venturi type. Accordingly, a great deal of initial cost is required and moreover, there is a disadvantage that the reaction pressure must be increased to compensate a pressure loss by such an extra apparatus.
The reaction product gas quenched by the above method forms a vapor phase and a condensed liquid phase, and methacrylic acid is separated and purified in steps as will be described next. Namely, methacrolein as a useful component is absorbed and separated, usually, with an absorbent such as water or an organic solvent from the vapor phase containing nitrogen, oxygen, carbon monoxide, carbon dioxide and steam. On the other hand, from the liquid phase composed principally of methacrylic acid and containing small amounts of aldehydes such as methacrolein together with formic acid, acetic acid, propionic acid, acrylic acid and water, methacrolein as a useful component is stripped, separated and recovered along with the other aldehydes. When methacrolein is separated and recovered as described above, solid matters often deposit in a diffusion column, thereby developing an operational problem such as interior blocking of the column. Basically, solid matters such as terephthalic acid are only sparingly soluble in the aqueous solution of methacrylic acid. Due to their slow precipitation velocities, the aqueous solution is however fed to the diffusion column before such solid matters have precipitated fully. The solid matters hence occur in the diffusion column and deposit there.
After stripping, separating and recovering methacrolein together with other aldehydes in the above-described process, methacrylic acid is separated in a purified form usually by extracting the liquid phase with a solvent capable of extracting methacrylic acid selectively and then removing formic acid, acetic acid, propionic acid, acrylic acid and water, which are contained in small amounts in the extract, by azeotropic distillation with a solvent. In this selective extraction of methacrylic acid, solids matters often deposit in an extraction column to cause a problem for the stable operation of the process such as interior blocking of the column. Basically, solid matters led by terephthalic acid are only poorly soluble in the aqueous solution of methacrylic acid. Due to the low precipitation velocities of the solid matters, the aqueous solution is however fed to the extraction column before the solid matters have precipitated fully. The solid matters hence occur in the column and deposit there upon contact of the aqueous solution of methacrylic acid with the extracting solvent, although the degree of their deposition varies depending on the kind of the extracting solvent. As conventional techniques for solving these problems, it has been known to bring the aqueous solution into contact with the solvent before feeding the aqueous solution to the extraction column and then to filter off solid matters thus occurred (Japanese Patent Laid-Open No. 16438/1981), to add a basic substance to the aqueous solution before feeding the aqueous solution to the extraction column, thereby causing the solid matters to decompose or to move as salts to the side of an extraction residue (Japanese Patent Laid-Open No. 99434/1983), and to add a bisulfite to the aqueous solution before feeding the aqueous solution to the extraction column, thereby preventing the solid matters from occurring in the extraction column (Japanese Patent Laid-Open No. 128337/1983), etc. The above-described methods may be effective in removing organic compounds, such as terephthalic acid, dissolved in the aqueous solution of methacrylic acid in a stage prior to the extraction of methacrylic acid. However, large facilities are required for the slow precipitation velocities of the organic compounds or a special chemical is required. They were therefore unable to provide a perfect solution to the problem of deposition of solid matters inside an extraction column.